1. Field of the Invention
The present invention relates to pipe joints and valves and, more particularly, to a device for imparting a live-load to gaskets or packing material for improving the seal around pipe joints and valves.
2. Description of the Background
There are thousands of valves and pipe joints operating in the numerous piping systems both inside and outside of nuclear power plants. The need for a proper seal in such pipe joints and valves is self-evident. If leaks develop, they may well result in radioactive exposure of personnel. Furthermore, steam production is quite expensive, costing approximately $3.40 per 1000 pounds. A small leak in a single steam valve can waste over $5000.00 in energy costs per year.
Such leaks are typically avoided by employing gaskets between pipe joints and resilient packing material around valve stems. Unfortunately, conventional gasket and packing material gradually lose resiliency over time and become permanently deformed. Permanent deformation (a.k.a. "creep") of the gasket or packing material must be compensated for by maintenance procedures including routine tightening of the studs clamping the gasket or packing material. The studs are tightened regularly until all resiliency is gone, at which point the packing material must be replaced. These maintenance procedures have proven to be time-consuming, costly, and a dangerous source of radiation exposure in radioactive areas.
It is well-known that a spring device can be used to compensate for creep, thereby eliminating the need to tighten the studs regularly over the life of the packing material. The spring device imparts a constant compressive force to the gasket or packing which compensates for the resiliency normally lost to creep. The seal provided by the packing or gasket will remain stable until the time when replacement is required. Consequently, much of the maintenance is eliminated. The use of a spring-device as described above is known as "live-loading" pipe joints and valves.
A commonly used spring device for imparting a live-load is known as the Belleville washer stack (BWS).
FIG. 1 illustrates a prior art live-load system for a valve. The system incorporates a Belleville washer stack 90.
As shown in FIG. 1, the live-load system includes a valve housing 10 having a central valve chamber 20. A valve stem 30 projects through valve chamber 20. An upper portion of the valve chamber 20 has a radius which is larger than the radius of the valve stem 30. This leaves a clearance between the valve chamber 20 and valve stem 30 for accommodating a layer of packing material 40 encircling valve stem 30. An annular gland collar 50 likewise encircles valve stem 30 above packing material 40. Gland collar 50 is slidable along valve stem 30 for compressing packing material 40 within valve chamber 20. An annular gland flange 60 also encircles valve stem 30 above housing 10. Gland flange 60 is likewise slidable along valve stem 30, and bears against gland collar 50 in order to force gland collar 50 against packing material 40. Flange 60 is formed with a number of bore holes spaced evenly around the periphery. A corresponding number of gland studs 70 are threaded into valve housing 10, each protruding upward, in parallel to valve stem 30, through a bore hole in flange 60.
A stack 90 of individual Belleville washers 100 are mounted on stud 70, and a gland nut 80 is screwed to the tip of each stud 70. Each stack of washers 90 is sandwiched between a gland nut 80 and flange 60.
FIGS. 2 and 3 illustrate a top view and a side view, respectively, of an individual Belleville washer 100 of the type commonly used in a live-load system as in FIG. 1.
As shown in FIGS. 2 and 3, each Belleville washer comprises a convex annular disk having a central aperture 110.
Referring back to FIG. 1, pairs of opposing disks 100 are aligned top to bottom on studs 70 to form a resilient Belleville stack 90.
In operation, stack 90 imposes a spring force on flange 60, which in turn urges gland collar 50 into packing material 40, thereby compressing the packing material 40 within valve chamber 20.
The above-described "live-load" system provides a superior seal. Moreover, the compressive force on packing material 40 is uniformly maintained over time to accommodate creep.
Unfortunately, the standard dimensions of valves and conduits often make installation of the Belleville washer stack 90 difficult or impossible. It is necessary to replace certain valve parts with custom parts to accommodate a Belleville spring device. Moreover, the proximity of studs 70 to valve stem 30 leaves little room for a Belleville washer 100. Valves smaller than two inches cannot be outfitted with Belleville washers due to a lack of space around studs 70. For larger valves, various sizes of Belleville washers must be kept on hand to fit each of the standard valve sizes. In addition, different valves require different spring constants. Hence, the Belleville washer stack 90 must be customized in accordance with the valve type to provide the proper spring constant.